Thanks for this guys,
The msbuild plugin (same for mstest) are packaged as zip files. The manual upload through the Jenkins UI chokes on this type of file because it isn't a *.hpi file. I had tried just copying the unzipped package directory to the plugins directory but, without a matching .jpi file for that plugin, Jenkins didn't recognize having to load it.
Last night I just used my personal system to load the plugins through the standard method of using the Available tab and checking the ones I wanted. Somehow, that method is able to load those packages correctly. Copying the directory and matching .jpi files onto my work system then worked fine.
MW